1. Field of the Invention
The present invention relates to a LED Light Emitting Diode (LED) Planar light source, and more particularly to a LED planar light source which introduces blue light or ultra violet rays emitted from luminescent crystals to a light conductive plate from its side surface. After being scattered and diffused by the light conductive plate and reflected from a reflection layer, the incident light and the excited light from a fluorescent pigment layer are synthesized to form white light or light of other specified wave lengths. Such a planar source can be utilized as a back light source for a Liquid Crystal Display (LCD).
2. Brief Description of the Prior Art
Planar light sources are mainly utilized as back light sources for LCD's. They can be classified according to their configuration as follows:
(1). Miniature tubular cold cathode fluorescent lamp (CCFL) with a diameter of about 2 mm, incorporation a light conductive plate and a reflection layer for diffusing the light. Its lifetime is about 10,000 hours. It is disadvantageous in that a high voltage and frequency are necessary to drive it. PA1 (2). Electro luminescence (EL), is advantageous with a thin structure, but on the contrary, it is disadvantageous since it has to be driven by high voltage and has a short lifetime of about 3,000 hours.
(3). Light emitting diode, (LED) with a long lifetime of more than 50,000 hours and no need of being driven by high voltage, has its advantages, but it is disadvantageous in that a plurality of LED elements must be used to produce sufficient luminous fluxes, as its individual luminous intensity is too small. LED is generally utilized as a small area back light source for black and white LCD.
A Sheet-Like Light Source' was published in the Japanese Patent gazette on Dec. 1, 1996, in which a light emitting diode is used and which can be used as back light and can emit white light by arranging a blue light emitting diode, a diffusing layer and a transparent film having a yellow fluorescent material of yttrium aluminum garnet (YAG) on a transparent light conductive plate.
As shown in FIGS. 1 and 2, there are no basic differences between the construction for the aforementioned white back light plate and other traditional LED planar light sources. The main structure comprises an upper diffusing layer, a light conductive plate as the middle layer, and a reflection layer as the lower layer and a base. It is characterized in that blue light emitting diodes are used as the light source. A layer of yellow fluorescent pigment is applied on the inner surface of the upper diffusing layer, and by means of mixing and synthesizing the blue and yellow lights, the desired white light can be obtained.
However, such arrangement for the components of the planar light source has several disadvantages in that the fluorescent pigment applied on the surface of the planar light source may interfere with the passing of light and it is difficult to control the uniformity of the fluorescent pigment layer, thereby resulting in uneven light color. It is well understood that for a fluorescent pigment itself, it is not a good arrangement to excite a fluorescent layer with incident light from the back and emitting the excited fluorescent light from its front because a low efficiency of light emission will result. Yet at present, construction of all kinds of televisions and fluorescent lamps belong to this category.